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In chemical science, dialysis refers to the process of separation of suspended colloidal particles from dissolved ions or molecules of small dimensions (Figure 1). Separation occurs due to their unequal rates of diffusion through the pores of a semipermeable membrane, such as dialysis tubing.

Figure 1. Illustration of process of dialysis
Figure 1. Illustration of process of dialysis

The process of dialysis was first observed in 1861 by a British chemist, Thomas Graham. He used this technique for the separation of smaller molecules (sucrose) from the larger ones (gum Arabic). He classified solutes as crystalloids or colloids.

Dialysis is used for isolation and purification of proteins. It is now also used for modern biophysical techniques and separation procedures. For example, dialysis is used concurrently with nuclear maganetic resonance (NMR ) or optical dispersion to study antibody peptides structures in solution.

Principle

There are three processes are involved in dialysis

  • Diffusion
  • Ultrafiltration 
  • Osmosis

Diffusion

Diffusion is the net directional movement of molecules occurring from a solution of higher concentration to a solution of lower concentration (Figure 2). The rate of movement in diffusion depends upon the concentration gradient of solute between two solutions, permeability of membrane to the solute, and surface area of the membrane.

Figure 2. Schematic representation of process of diffusion
Figure 2. Schematic representation of process of diffusion

Ultrafiltration

Ultrafiltration  is the movement of solvent across a semipermeable membrane in response to a pressure difference applied across the membrane.

Ultrafiltration is a selective fractionation process utilizing pressures up to 145 psi (10 bar). It is a membrane filtration technique, in which liquid is forced across a semipermeable membrane by applying hydrostatic pressure. It concentrates suspended solids and solutes of molecular weight greater than 1000 daltons. Solutes and suspended solids pass across the membrane. Permeate contains low-molecular-weight organic solutes and salts.

Recovery of an ultrafiltration system is defined as percentage of feed (liquid to be treated by ultrafiltration) water converted to permeate.

Where,
R= recovery
P = volume of permeate
F = volume of feed

The factors affecting the performance of ultrafiltration system are the operating pressure, and temperature, and the flow velocity of the molecules across the membrane.

Osmosis

Osmosis is the movement of solvent (e.g. water) from low concentration to high concentration across a semipermeable membrane (Figure 3).

Figure 3. Diagram representing the process of osmosis
Figure 3. Diagram representing the process of osmosis

The components of solutions being dialyzed are subjected to the separation process based upon their differential diffusion rates through a semipermeable membrane. Diffusate is the solution from compartment, which originally had lower concentration of solute of interest while; retentate is the solution from compartment having higher solute concentration. Traditionally, a dialysis cell consists of two compartments separated by a semipermeable membrane. The membrane chosen for the study is permeable to smaller molecules and impermeable to large molecules. With time, concentration of smaller solute will decrease in retentate and increase in diffusate, while large molecules remain in the retentate. Fick’s law of diffusion describes net movement of solute through the membrane.

Where,  = solute membrane diffusion coefficient

= concentration gradient across the membrane

 = cross-sectional area of the membrane

 is defined as total area of the membrane times the fraction of the membrane available for solvent movement.

In an ideal dialysis cell, the concentration gradient across the membrane is equal to the difference in concentration between the two compartments.

Factors Affecting Rate of Dialysis

Being a diffusion process, Dialysis follows Fick’s first law of diffusion, which states that the rate of transfer of molecules or atoms by diffusion through a unit area is proportional to concentration gradient.

Diffusivity or diffusion coefficient is directly proportional to the temperature and inversely proportional to the viscosity and the molecular volume.

Study of diffusion coefficient of proteins and peptides under different conditions provides information about changes in molecular volume. Diffusion across membrane also depends on pore size of membrane.

Other factors affecting dialysis rates are buffer composition and volume, number of buffer changes, and particle size of the solute.

Applications of Dialysis

Dialysis is used in research in various fields such as chemistry, biochemistry, medicine, and pharmaceutical sciences.

  • Dialysis is traditionally used for solvent exchange and removal of low molecular weight contaminants.
  • It is used for estimating molecular weights of active compounds, detecting solute heterogeneity, and isolation of substances from blood.
  • It has also been used extensively for binding studies (binding of small solute molecules to large solute molecules).
  • Dialysis is used for studying proteins or peptides and their conformation.
  • Dialysis is used in the patients with renal failure (Figure 4). It has other medical applications, such as apheresis (used to removed blood constituents), hemodialysis, or peritoneal dialysis. The artificial kidney uses cellulose membranes instead of phospholipid bilayer membranes used by real kidneys to separate the components of blood.
Figure 4 .Dialysis medical procedure system
Figure 4 .Dialysis medical procedure system

Hemodialysis uses a cellulose-membrane tube immersed in a large volume of fluid, while Peritoneal dialysis uses the lining of the patient’s abdominal cavity, called the peritoneum, as a dialysis membrane. Thus, artificial kidney dialysis system uses same basic principles as natural kidneys to maintain chemical composition of the blood. Diffusion across semipermeable membranes, polarity, and concentration gradients across the membrane are main features of the dialysis process for both natural and artificial kidneys.

Reverse Dialysis

Reverse dialysis is a technique for concentrating macromolecules in solution. The (dilute) solution is placed in a bag made of a semipermeable material, through which only water and small molecules can pass but not the macromolecules. The bag is placed in a bed of dry water‐soluble polymer (which cannot enter the membrane), such as polyethylene glycol. Water, together with other small molecules, is then drawn out of the bag into the external phase until equilibrium is reached, thereby concentrating the solution of macromolecules in the bag.